Henrik:
Amy's original question was not confined to audition or the
hearing threshold.

I am fully aware of this, I just wanted to mention that these low
frequencies actually can be heard.

The experiment you mention seems to be a (whole-)body vibration, not a
sound exposure and that is a completely different matter. (By the way,
have you seen a proper scientific reference?). Our sensitivity to
whole-body vibrations is highest at a few hertz, which e.g. is
reflected in the weighting curves in ISO 2631 "Mechanical vibration and
shock - Evaluation of human exposure to whole-body vibration".

For vibrations there are certainly a variety of resonance frequencies
in the body, and quite many are below 20 Hz. There are also body
resonances for acoustic exposure, but they are a bit higher in
frequency than for vibrations, since the body is a lot stiffer for
acoustic stimulation than for vibrations.

Henrik

The 7 Hz phenomenon that I referred to
was probably a fairly large amplitude, but I can't relate
it to "hearing threshold" at that frequency (!). As I
recall, subjects were in a jet pilot's seat atop a shaker
system. The 7 Hz sensitivity was assumed to be purely
visceral, having nothing whatsoever to do with audition...
probably just the effects of mechanical stretching and
scrubbing together of internal organs due to large motions
at the visceral resonance frequency. I have no doubt that
there would have been some synchonization of brain waves,
as a side effect of having one's guts tossed around!
Best regards,
Bob Masta
D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Signal Generator
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